FAQ02129 of Solid-state Relays FAQ

Countermeasures against DC Switching Output Noise Surges

Connect a diode to prevent counter-electromotive voltage if a load such as a solenoid or electromagnetic valve is connected. The output elements of the SSR may be damaged if a counter-electromotive voltage is applied that exceeds the withstand voltage of the SSR output elements.

As a countermeasure, insert the elements in Figure 1 in parallel with the load. (Refer to the following figure.)

The diode is the most effective method among absorbers to suppress counter-electromotive voltage. Solenoids and electromagnetic valves, however, have a long release time. Confirm operation with the actual circuit before using the SSR. Also, a diode and a Zener diode can be used as a measure to shorten the release time. If used, the release time will shorten as the Zener voltage (VZ) of the Zener diode increases.

Reference Information:

1. Diode Selection

Withstand voltage = VRM ≥ Power supply voltage × 2
Forward current = IF ≥ Load current

2. Zener Diode Selection

Zener voltage = VZ < (SSR collector-emitter voltage) - (power supply voltage + 2 V)
Zener surge power = PRSM > VZ × Load current × Safety factor (2 to 3)

Note: The capacity of the Zener diode (PRSM) increases as the Zener voltage (VZ) increases.